Where is Particle Velocity and Rotation in Blender 2.7a?
Velocity is a setting category for both emitter and hair types of particles. If you are using the hair type of particles, click on the advanced check mark (Under the type drop down box). If you are using the emitter type then you should see it by default.

For random looking grass, I would recommend you use Brownian under the Physics category then Newtonian type.